1. Field of the Invention
This application relates generally to a method and apparatus for minimizing a startup time of a gas turbine, and more specifically to method and apparatus for controlling a Wobbe Index of a fuel supplied to the gas turbine to minimize delays in turbine loading to allow the fuel to be heated before being delivered to the gas turbine.
2. Description of Related Art
Industrial turbines are often gas-fired and are commonly employed to drive loads such as generators installed at electric power plants to produce electric energy. Such gas turbines are designed to burn a specific range of fuels, and to burn such fuels under specific conditions, such as fuel temperature. The fuel composition and temperature are used to calculate a term know as the Wobbe index which speaks to the heating value of the fuel on a volume basis. The range of fuel flow from no load to base load, as well as the Wobbe index, are used to determine the fuel staging, or modes, within the combustor and other details such as the nozzle sizing.
In order to achieve higher plant efficiency at loads where continuous operation is expected, the fuel may be heated to a specific temperature. The heating of the fuel lowers the Wobbe index as the higher temperature increases the volume of the fuel at the same pressure. Operation of the turbine at a Wobbe index outside the design range may generate undesirable acoustic resonances that can damage the turbine hardware or result in the loss of flame in the combustor. In these cases, the elevated fuel temperature becomes a control permissive to enter these combustion modes to assure that the fuel has the required Wobbe index. If the fuel temperature has not been raised within the required range, then the system must hold at a given load until the temperature permissive has been met.
Gas turbines are also frequently called upon to drive large loads such as generators soon after a request for operation of the gas turbine is made. The gas turbine is expected to respond rapidly to a request for operation and quickly transition from an idle, or off mode in which the gas turbine produces no output power to a full load mode in which the gas turbine is driving the complete load per the operator's request. However, many gas turbine installations rely on their own hot exhaust gases or steam from a bottoming cycle to warm the fuel to be delivered to the combustor. Since it takes time for the exhaust gases to rise to their standard operating temperature the fuel initially delivered to the combustor immediately after startup of the gas turbine is initiated is lower than a target temperature of the fuel for which the gas turbine was designed. Introducing fuel below the target temperature can result in a Wobbe Index outside the desired range, possibly leading to combustion dynamics that can damage the gas turbine. And if a rapid response is required of the gas turbine to quickly drive a large load from startup, there will not be enough time to adequately preheat the fuel to an acceptable temperature to satisfy the demands of that specific load. Thus, the gas turbine is forced to burn the fuel below its target temperature and risk damage or experience a delay during startup, which may not be an option depending on the load to be driven.
To avoid a scenario where the gas turbine burns insufficiently-heated fuel at startup some installations including gas turbines also include a separate, stand-alone boiler to heat the fuel before it is delivered to the gas turbine at startup. However, to meet the rapid response startup demands of a large load as mentioned above the auxiliary boiler must be maintained in an active, fired state, and on standby at all times, even when the gas turbine is not actively operating. Continuously maintaining such a boiler on standby at all times consumes significant amounts of energy, is costly, and wasteful.
Accordingly, there is a need in the art for a method and apparatus for minimizing the fuel effects during a transition of a gas turbine from a first loaded state to a second loaded state where the load being driven is greater than in the first loaded state. Such a method and apparatus can optionally adjust a Wobbe Index of a fuel, at least temporarily, until such time as the temperature of the fuel can be sufficiently elevated to satisfy a change in load on the gas turbine. The method and apparatus can optionally adjust the Wobbe Index of the fuel based on a composition of the fuel, and can optionally adjust the Wobbe Index in this manner in parallel with heating of the fuel.